Disk drives include one or more disks on which digital information is stored as magnetic charges. The disk (or disks) is mounted on a spindle rotated by a spindle motor. An actuator assembly includes an actuator arm and a voice coil motor (VCM). The actuator arm extends from the VCM and supports a slider that includes a read/write head. The head reads from and writes to the disk as the slider flies over the disk on an air cushion. The VCM positions the head at desired locations relative to the disk.
Disk drives have been designed with a landing zone at the inner diameter of the disk to park the head. The landing zone is a takeoff or landing spot for the head as the disk starts or stops spinning, respectively. As the disk starts spinning, the head is dragged on the disk until the disk reaches a speed that creates sufficient air pressure for the head to separate from and fly over the disk.
The disk can have a rough texture to minimize friction between the head and the disk in the landing zone. However, as disk drive storage capacity increases, the flying height of the head decreases, and the disk is given a smooth texture to avoid damaging the head. The smooth texture dramatically increases the contact friction between the head and the disk in the landing zone. As a result, increased spindle motor current may be required to break the head loose from the disk to allow the disk to rotate, or spin up.
Disk drives have increasingly smaller form factors, or disk sizes (2.5″, 1.8″ and 1″). Small form factors are useful in battery-operated devices where increased spindle motor current to spin up the disk is undesirable. Small form factors reduce the disk surface area, which is reduced further by a landing zone. Small form factors are also more susceptible to operational and non-operational shock if the head and the disk are in contact. Thus, small form factors are penalized by a landing zone.
Disk drives have been designed with a ramp to avoid a landing zone. The head is lifted off the disk and unloaded on the ramp while the disk is spinning, and then the disk decelerates and stops spinning. When power is reapplied to the spindle motor, the disk spins up, and once the disk has sufficient speed for the head to fly, the head is loaded from the ramp and positioned over the disk.
During ramp load/unload, the head velocity is accurately controlled to avoid damaging the head or the disk at a contact point. As the VCM moves through its magnetic poles, it generates a back electromotive field (EMF) voltage which is proportional to its speed. The back EMF voltage also indicates the head velocity. The back EMF voltage (Vbemf) can be calculated based on the total voltage across the VCM (Vvcm) and the IR drop across the VCM (Ivcm×Rvcm) as follows:Vbemf=Vvcm−(Ivcm×Rvcm)  (1)
Thus, the back EMF voltage is measured by removing the VCM IR drop from the VCM voltage.
The back EMF voltage can be measured by a pulse width modulation (PWM) technique or an IR cancellation technique. In the PWM technique, the VCM is turned off periodically, forcing the VCM current to zero. Since the IR drop across the VCM is zero, the back EMF voltage is readily measured. In the IR cancellation technique, the back EMF voltage is determined by measuring the gain of a servo loop. Since the VCM current is not periodically turned off, calibrations cancel the IR drop from the VCM voltage. The calibrations may need to be repeated because temperature and voltage deviations cause the gain of the servo loop to change frequently over time.
The PWM technique requires less hardware and fewer calibrations than the IR cancellation technique. However, the PWM technique may generate audible noise during ramp load/unload. The IR cancellation technique, however, requires robust calibration with more hardware. Further, increased voltage resolution may require a hardware change to increase the number of bits of the analog-to-digital converter.
Disk drives have been designed for either the PWM or IR cancellation techniques. If both techniques were needed, two distinct sets of hardware had to be implemented, thereby increasing cost. Further, the technique needed to be selected before any voltage measurements, thereby greatly reducing flexibility.
There is, therefore, a need for a disk drive with accurate control of the head during ramp load/unload. There is also a need for a disk drive that measures the back EMF voltage with either the PWM technique or the IR cancellation technique without hardwiring a specific measurement technique or making multiple calibrations prior to operation of the disk drive.